High charging currents can occur when uncontrolled rectifier circuits are connected to a power system with capacitive smoothing at the output, because generalized phase control is impossible. In addition, an oscillating circuit formed by the capacitor and the line inductances may be activated by the high charging current due to the fact that damping of the respective current path is usually relatively low, thus leading to high, short-term voltage surges which can have a harmful effect on various parts of the circuit arrangement.
Conventionally, to avoid these negative effects, an ohmic resistor is connected into the current path for the precharging phase of the smoothing capacitor, resulting in the charging current being limited to a level which depends on the applied line voltage. After a predetermined period of time has elapsed or when a predetermined voltage threshold has been exceeded at the smoothing capacitor, the ohmic charging resistor can then be short-circuited by a contactor. However, due to the inertia of a mechanical contactor, such a starting current limiter cannot become effective again until after the elapse of a release time of the contactor, which is manifested, for example, in failure of a line half-wave in increased charging currents.
Use of an NTC thermistor does not yield any permanent improvement here, because due to the thermal time constant, this also has a certain inertia which does not allow it to react to the failure of a line half-wave.
German Utility Model 92 16 662 describes a precharging circuit for the capacitor at the output of a power converter, where precharging of the capacitor can take place across an ohmic resistor through a diode network connected between the power system and a terminal of the smoothing capacitor in parallel to the power converter. As soon as the capacitor is charged, normal operation can be resumed with the actual power converter. However, the capacitor here is constantly connected to the power system by way of the diode network, so it is impossible to lower the intermediate circuit voltage, for example reduce it to a lower voltage with controlled valves. On the other hand, this circuit makes sense only in conjunction with controlled rectifiers, because otherwise the rectifier, which is connected without an ohmic resistor, would immediately assume the role of carrying the current when switched on and would cause unwanted high charging currents.